A thermal-hydraulic analysis methodology is established for a pressurized thermal shock (PTS) analysis of the Kori Nuclear Unit-1 (KNU-1) power plant using RETRAN-3D. The effects of the important parameters on PTS are evaluated, such as the initial power level, break size, isolation of the depressurized steam generator (SG), and charging flow runback. As a result, the most dominant factors are revealed as the initial power level and break flow rate. Auxiliary feedwater (AFW) to the depressurized SG should be isolated by operator action as soon as possible to minimize break flow. To mitigate the risk of PTS by reducing both steam flow rate and operator response time of depressurized SG isolation, the installation of check valves to block the cross flow between two steam lines is recommended. An efficient grouping methodology is proposed using these results. Groups should be divided according to the break size, the initial power level, and the AFW isolation to a depressurized SG. Charging flow runback is revealed to have little effect on downcomer overcooling.